With the advent of modern day electronics, the circuits on a PCB (Printed Circuit Board) or ceramic modules or chips have become more and more dense. As a result the electrical lines or conductor lines are thinner and narrower so that more of them can be placed in a given area. Therefore, the probability of the conductor lines having defects increases and each of the electronic line carriers has to be inspected for faults in the conductor lines. This inspection can be done visually (manually) or automatically, or the lines can also be electrically verified. Once a fault or defect is found, then it has to be located and repaired. Most of the methods presently under use require that the fault or defect be visually located by the operator, and then the defective conductor line is repaired.
The testing and repairing of the conductor lines are among the most critical steps in the packaging technology. This is because the electronic hardware must be reliable and free from defects, as they are very expensive to manufacture and the field failures cannot be repaired easily. To eliminate these immediate and potential defects, tremendous efforts are being made.
The most commonly found defects are opens, or cracks in lines, or shorts between lines. Most defects or faults in a conductor line are due to masking or improper deposition of the conductive material. But they could also be related to other factors, such as impure material or stretching the resolution limits of the lithography process. The thin film redistribution lines may have opens due to contamination, process mishaps and physical damage.
Another reason for opens is due to stresses generated during thermal cycles in the bond and test process used during assembly of PCB or ceramic modules. The thin film lines with cracks and other latent defects may also develop opens. These opens must be repaired in order to use the substrate or module or package which is otherwise electrically good.
Particularly, in the thin film processing, the thin film redistribution and other interconnection lines are susceptible to defects which could result in the lines being electrically open. Defects could include voids, missing metal, various particle contamination or physical damage. A redundant metal scheme helps to substantially reduce the number of defective lines, but this scheme does not eliminate them entirely. Those lines identified as "open" after thin film electrical test can be repaired by processes such as laser CVD (Chemical Vapor Deposition) prior to a polyimide overcoat process. Occasionally a line becomes "open" during subsequent thermal processing. These defective conductors appear in the module or substrate, after pins, capacitors and chips are joined. If an "open" line should be found at this point, it is essential to repair the defect so that the module or the substrate or the package can be used.
It is disclosed in, "Repairing Breaks in Printed Circuits," IBM Technical Disclosure Bulletin, Vol. 8, No. 11, Page 1469 (April, 1966), that small breaks in a line can be repaired by filling the gap in the broken line with a material that is cured at room temperature or higher to form a base conductive material. A conductive metal layer is then electroplated over the base conductive material to complete the repair. Using this process would require that lines to be repaired, on extremely dense packages with chips, capacitors and discrete wires in place, be isolated during electroplating. This would create significant handling and tooling problem.
"Open Conductor Repair For Glass Metal Module," IBM Technical Disclosure Bulletin, Vol. 14, No. 10, Page 2915 (March, 1972), discloses another method of making open repairs. Here a metal line to be transferred is aligned over the open or break, and using a laser beam, a portion of the metal layer is welded to each end of the broken line. This article also teaches that the line could be reflowed into the break using a laser or it could be evaporated into the break. Each of these features cannot be used with the present invention, because melting of high temperature conductive metals, such as copper, is used. Energy required to melt such lines by laser would damage polyimide adjacent to the lines to be repaired.
A rather complex process for repairs of opens is disclosed in U.S. Pat. No. 4,259,367 (Dougherty, Jr.), where a conductor patch line is interconnected onto a good line through an insulating layer. This requires the addition of new wiring layers with photolithographic techniques which would be incompatible with a substrate with components already in place.
Another method of repairing opens is by decal transfer as disclosed in U.S. Pat. No. 4,704,304 (Amendola, et al.), and presently assigned to IBM Corporation.
Still another method of electric circuit line repairs is taught in U.S. Pat. No. 4,630,355 (Johnson). A layer of phase-change material is deposited prior to the deposition of the conductive line and in case an open results in the conductive line, a current is passed so that a portion of the phase-change material becomes electrically conductive and makes an electrical bridge across the gap or open. This method is not suitable for repairs on polyimide films due to lack of adhesion of such phase-change materials to polyimide.
In U.S. Pat. No. 4,418,264 (Thorwarth), a specifically shaped metallic part is placed on the conductor path interruption and by means of microresistance welding, the metallic part is welded to the conductor to bridge the interruption. Welding involves melting of the repair material which when used on current "state of the art" thin film polymer packages could cause structural damage to the polymer. Welding also requires the passage of drive currents which would be incompatible with this invention as there may be active devices which are connected to the lines being repaired at different locations.
Another method of repairing opens and narrow necks has been disclosed in U.S. Pat. No. 4,919,971 (Chen). The defective site in the conductor line having a thin portion or a narrow neck does not have to be physically located to initiate the repairs. The process of this invention is self-induced, i.e., the passage of the drive current creates a hot spot at the defective site and conductive material is induced to be deposited at the defective site. The process of this invention is also self-limiting, i.e., when the defect has been repaired, the process will slow down and stop by itself. This technique requires the substrate to be immersed in a plating bath or be exposed to organometallic vapors which would make it incompatible with line repair processes where the active and passive components have already been mounted on a substrate.
"Conductive Line Jumper/Repair Connection in Glass Metal Module," IBM Technical Disclosure Bulletin, Vol 15, No. 8, Page 2423 (January, 1973), discloses another method of making open repairs. Here after the open has been located, a wire is placed across the open line and the wire is welded to each end of the open line. After welding, the repaired plane is glassed over leaving a surface suitable for developing another circuit layer. This process teaches the repairs of the carrier at the build level, and not at the functional module level. This process also requires the use of high melting point metals and a subsequent sintering of inorganic materials.
Another welding process for repairing of opens is discussed in, "Circuit Repair/Work of Metallized Polyimide Substrates," IBM Technical Disclosure Bulletin, Vol. 22, No. 9, Page 3986 (February, 1980). A piece of wire is jumpered across the open and both ends of the jumpered wire are welded to the circuit line, thus yielding a "continuous electrical line." This process also discloses the use of high melting point metals.
Another method of making circuit repairs is disclosed in, "Tailless Thermo-Compression Bonding," IBM Technical Disclosure Bulletin, Vol. 27, No. 5, Page 3041 (October, 1984), where the circuit line is repaired by passing an electric current between two electrodes which fuse the circuit line and the repair material together.
"Josephson Package Repair," IBM Technical Disclosure Bulletin, Vol. 26, No. 12, Pages 6244-6245 (May, 1984), is another example of making repairs. The faulty circuits are cut out by laser scribing, and the repair of an open is done by cutting the bad line next to the pad and using a third wiring level to reconnect to the proper pads. This process has the limitation of requiring photolithographic techniques to form the new wiring level. Furthermore, additional thin film process steps cannot be done after chips, pins etc, have been attached.
Laser deposition methods are also being developed for repairing circuit opens. As disclosed in U.S. patent application Ser. No. 223,487, filed on July 25, 1988, now U.S. Pat. No. 5,182,230, and presently assigned to IBM Corporation, an open circuit is repaired by laser induced electroplating process based on the thermobattery effect. One tip of the open conductor is heated with a laser beam, and a thermobattery is formed between the hot spot (tip of the conductor) and the cold part (normal section of the conductor). The laser heating of the tip induces the conductive material present in the plating solution to be formed at the hot tip. This process is continued until the growth of the conductive material joins the two open ends of the open, and a continuous electrical path is formed.
Another process for interconnecting thin-film electrical circuits is taught in U.S. Pat. No. 4,880,959, and presently assigned to IBM Corporation. Both ends of the existing circuit are partially ablated at the open defect site with pulses from an excimer laser, and then gold metal is deposited by LCVD (laser chemical vapor deposition). This process makes the repairs right after the thin film deposition, and prior to any subsequent module build (i.e. at the substrate level).
Under some circumstances a laser, as disclosed in U.S. Pat. No. 4,572,941 (Sciaky, et al.), could be used to make spot welds. The laser induced melting can cause structural damage to sensitive dielectrics and adjoining lines.
Thin film metallurgical lines are often applied or deposited onto a dielectric material which acts as a carrier or substrate for the deposited features. The sole purpose of this carrier may be the physical support of the thin film features as is the case of most dielectric polymers. The carrier may also serve as part of the over-all electrical package in which thin film lines are deposited directly onto a single or multilayered ceramic substrate.
The thickness, width and pitch of a thin film line is dependant upon the desired electrical properties of the package. Often a single thin film line will vary in shape, width and proximity to adjacent features along its total path. As a result, process defects may result in opens across these thin film lines, as discussed earlier, at a point where the geometry of the line and the distance to the nearest feature could be very close. Again, the distance to the nearest feature vary widely from line to line.
The use of Laser Chemical Vapor Deposition (LCVD) to repair opens on thin film lines requires that process parameters be established which take into account heat transfer at the repair site.
Typically, when electrically conductive material or metal, such as, gold, copper, etc, is deposited to bridge the open on a thin film line it leads to excess metal deposition on adjacent features if the distance between features is very close, say for example, less than 50 microns. In addition, once the bridging is formed, laser energy starts to dissipate away from the line to be repaired and starts entering the adjacent lines that got connected. The latter yields poor deposition on the repair site.
The process of this invention guards against both situations. Basically, the location to be repaired is located by methods well known in the art. After the open or location to be repaired has been site dressed, at least one blind hole or groove is formed in the material surrounding the open, typically, using an excimer laser. For best results two grooves or trenches or blind holes should be formed on the left and right side of the open or location to be repaired. These grooves solve the two problems described above by directing more of the laser energy along the path of the defective line. As a result, more electrically conductive metal, such as, copper or gold, is deposited across defect in the line, leading to more reliable repairs. By directing the heat along the thin film line being repaired, less heat is allowed to migrate towards other features, thus eliminating the possibility of inadvertent shorting.